10. The Reticular Formation and Consciousness Flashcards
Define consciousness.
Define consciousness.
awareness of external environment and internal states
Define arousal.
Associated with goal-seeking behaviour and avoidance of noxious stimuli
What 2 neural components are required for consciousness?
Cerebral cortex and reticular formation
What are the cortex and reticular formation connected by and what does it form?
Connected by reciprocal excitatory projections forming a positive feedback loop
- binary outcome
- awake/not awake etc
what is the role of cerebral cortex in consciousness?
the site where conscious thoughts arise
Receives many inputs, including from the reticular formation
what is the role of reticular formation in consciousness?
the circuitry that keeps the cortex ‘awake’
What are 2 important inputs to the reticular formation?
Cortex and sensory systems
What is the reticular formation?
A population of specialised interneurones in the brainstem
What are the 3 major output pathways of the reticular formation and what neurotransmitter is involved?
Cholinergic (excitatory fibres) to:
- basal forebrain nuclei
- hypothalamus
- thalamus
- (also down to the spinal cord, maintaining muscle tone)
What type of fibres project from the basal forebrain nuclei ?
excitatory cholinergic fibres to cortex
- (think sedative side effects of anticholinergics)
What type of fibres project from the hypothalamus?
excitatory histaminergic fibres to the cortex
- (think sedative side-effects of sedating antihistamines)
What type of fibres project from the thalamus?
excitatory glutamatergic fibres to the cortex
What is the part of the reticular system involved in arousal called?
Reticular activating system
What is used for clinical assessment of consciousness?
Glasgow Coma Scale (GCS)
What are the components of GCS?
- eye opening
- motor response
- verbal response
How is Eye Opening scored on GCS and what do they suggest?
- Spontaneous eye opening (4) suggests normal cortical and brainstem function
- Response to speech (3) suggests slightly diminished cortical function but still functioning brainstem
- Response to pain (2) suggests impaired cortical function but brainstem preserved so that reflex opening can occur
- No response (1) suggests severe damage to brainstem +/- cortex
How is motor response scored on GCS and what do they suggest?
- Obeys commands (6) suggests normal function with working connections from auditory system to brainstem/cord
- Localises to stimuli (5) suggests diminished higher cortical function but still connections working from sensory to motor cortex
- Withdraws to pain (4) suggests that there is still a ‘physiological’ reflex response to stimuli
- Flexor response to pain (3) suggests a lesion above the level of the red nuclei. This response is still ‘semiphysiological’
- Extensor response to pain (2) suggests a lesion below the red nuclei. This response is not physiological at all
- No response to pain (1) suggests severe damage to brainstem +/- cortex
How is verbal response scored on GCS and what do they suggest?
- Oriented in time/place (5) suggests normal cortical function
- Confused conversation (4) suggests diminished higher cortical function but language centres are still functioning adequately
- Inappropriate words (3) suggests language centres have been damaged
- Incomprehensible sounds (2) suggests cortical damage with brainstem mediated groans
- No response (1) suggests severe damage to brainstem +/- cortex
What is the electroencephalogram?
Measures the combined activity of thousands of neurones in a particular region of cortex
- high temporal resolution
- low spatial resolution
What are electroencephalograms good for detecting?
Good for detecting neuronal synchrony, and evidence of normal cerebral function
How many stages of sleep are there?
4 major stages + REM sleep
How does awake with eyes open present on an EEG?
Beta waves - irregular, 50Hz
How does awake with eyes closed present on an EEG?
Alpha waves - regular, 10Hz
How does stage 1 sleep present on an EEG?
Background of alpha + interspersed theta waves (theta at around 5Hz, regular)
How does stage 2/3 sleep present on an EEG?
Background of theta + interspersed sleep spindles and k-complexes:
- Sleep spindles are high frequency bursts arising from the thalamus
- K-complexes represent the emergence of the ‘intrinsic rate’ of the cortex
How does stage 4 sleep present on an EEG?
Delta waves - regular, 1Hz
- Related to k-complexes seen in stages 2/3
How does REM sleep present on an EEG?
- EEG similar to beta waves
- Dreaming occurs in this stage, so similar to the EEG in a conscious patient
What is the neural mechanism of (non-REM) sleep and what helps facilitate it?
- Complex
- Deactivation of the reticular activating system (and hence cortex) + inhibition of the thalamus
- This deactivation is facilitated by removal of sensory inputs (fewer positive influences on positive feedback loop)
What is REM sleep initiated by?
Initiated by neurones in the pons (i.e. initiation appears to be an active process)
What is the EEG of REM similar to, why is it difficult to wake a person in REM sleep?
- similar to awake, eyes open
- difficult to rouse due to strong thalamic inhibition
What is the muscle tone during REM sleep and why?
Decreased muscle tone due to descending inhibition of LMNs by glycinergic fibres arising from the reticular formation and running down the reticulospinal tracts
What cranial nerve functions may be preserved during REM sleep?
- Eye movements
- teeth grinding (nocturnal bruxism)
What are some autonomic effects during REM sleep?
- Penile erection,
- loss of thermoregulation (homeless hypothermia)
What does long term deprivation of REM sleep lead to?
Death
What are some functions of sleep? (4)
Generally unknown
- Energy conservation / repair?
- Memory consolidation?
- Clearance of extracellular debris? (bulk movement of interstitial fluid into CSF)
- ‘Resetting’ of the CNS?
Give 3 examples of sleep disorders.
- insomnia
- narcolepsy (may be caused by mutations in orexin gene)
- sleep apnoea (daytime sleepiness)
what is insomnia commonly caused by?
underlying psychiatric disorder as opposed to ‘primary’ insomnia
what is sleep apnoea commonly caused by?
excess neck fat leading to compression of airways during sleep and frequent waking
excess neck fat leading to compression of airways during sleep and frequent waking
- locked in syndrome
- persistent vegetative state (PVS)
- coma
- brain death
